Wireless power transfer is the transmission of electrical energy from a power source to an electrical load without the use of conductors, such as interconnecting wires. Wireless power is a generic term that refers to a number of different power transmission technologies that use time-varying electric, magnetic, or electromagnetic fields. In wireless power transfer, a wireless transmitter connected to a power source transmits field energy across an intervening space to one or more receivers, where it is converted back to an electric current and then used. Wireless transmission is useful to power electrical devices in cases where interconnecting wires are inconvenient, hazardous, or are not possible. However, current wireless power transfer systems suffer from inefficiencies related to dimensional-tolerance effects on performance.